This invention relates to a lithography process using a non-optical exposure apparatus, and more particularly a cell projection mask used for an exposure mask in the lithography process using a non-optical exposure apparatus.
Non-optical exposure apparatus using electron beam, ion beam or X-ray or the like as a light source has a superior resolution to the conventional optical exposure apparatus using G-line or I-line as a light source. According to this, the lithography process using the non-optical exposure apparatus, for example the electron beam lithography process using electron beam as a light source makes it possible to form fine patterns of below critical dimension which are not obtained by the prior lithography using the conventional optical exposure apparatus. The electron beam lithography process is typically utilized in fabricating a mask for exposure as well as in fabricating non-memory devices such as a logic device or an ASICs.
Recently, the lithography process using a cell projection mask as a mask for exposure is developed. It has advantage in productivity and it is applicable to fabricate memory devices. Because the cell projection mask is applicable to all non-optical exposure apparatus using electron beam as well as X-ray or ion beam as a light source, a study on the cell projection mask has been variously accomplished.
FIG. 1 is a sectional view of a cell projection mask in the prior art. A cell projection mask 10 includes a frame 1 for supporting means, a membrane 2 formed over the frame 1, for making a stress due to electron beam to maintain balance and an absorber 3 formed over the membrane 2, for absorbing or reflecting electron beam. The reference numeral 4 designates apertures for transmitting electron beam. So as to fabricate the cell projection mask 10, a silicon-on insulator (SOI) wafer where an oxide layer is sandwiched between silicon layers is used.
Herein, the silicon layer for the absorber 3 should be maintained at a thickness of 10-20 .mu.m and the reason is as follows. FIG. 2 shows a sectional view illustrating electron scattering in a silicon layer in case where the silicon layer is used for the absorber 3. An electron beam is projected into the silicon layer 30, electrons of the electron beams collide with silicon atoms of the silicon layer 30 to generate secondary electrons and the secondary electrons are scattered. While electrons are continuously scattering, if the induced force is removed, then the electron scattering is stopped. The reference numeral 20a designates the scattering course of electrons.
The depth where electrons are scattering is called as a penetration depth, which is determined by an incident energy of the electron beam. In general, the penetration depth of the electrons is about 1 .mu.m in case of the incident energy of 10 keV and about 4 .mu.m in case of 25 keV and about 12 .mu.m in case of 50 keV. Accordingly, in the lithography process requiring an incident energy of the electron beam at 50 keV, so as to prevent the electron beam from penetrating, the silicon layer for an absorber should have a thickness of about 12 .mu.m. Accordingly, considering the resolution, it should maintain the silicon layer to be at a thickness of 10-12 .mu.m.
When the thickness of the silicon layer is 10-12 .mu.m, the silicon layer as an absorber prevents the electron beams from penetrating. However, it is difficult to etch the silicon layer having a thick thickness of 10-12 .mu.m and it takes a long time to etch the silicon layer, so that it is undesirable in productivity. Considering the process aspect, although it should reduce the thickness of the silicon layer, the electron beams penetrating the silicon layer for an absorber are increased and the resolution is remarkably degraded. Furthermore, if the thickness of the silicon layer is thicker, the electron beam reflected in the sidewall of the absorber made of silicon layer scatters the electron beam passing through the aperture. As a result, owing to the mutual interference, the contrast of the electron beam becomes low, so that it is impossible to form the desired pattern.